Multi-filament composite superconductor with transposition of filaments and method of making same

ABSTRACT

A multi-filament superconducting composite composed of a plurality of segments that is intrinsically stable. The individual segments are multi-filament composites that have been twisted after assembly and mechanical working and thereafter subsequently deformed. The deformed segments can be triangular or rectangular and are assembled into a second composite. After assembly the second composite is once again twisted. This second twisting transposes the filaments within the segments, thereby producing a superconductor that is resistant to flux jumps induced by self-field losses.

United States Patent [191 Critchlow et a1.

[451 Aug. 20, 1974 MULTI-FILAMENT COMPOSITE SUPERCONDUCTOR WITHTRANSPOSITION 0F FILAMENTS AND METHOD OF MAKING SAME Inventors: PhilipR. Critchlow, St. Bruno,

Quebec, Canada; Bruce A. Zeitlin, North Plainfield, NJ.

Assignee: Airco, Inc., New York, NY.

Filed: Sept. 11, 1973 App]. No.: 396,282

Related [15. Application Data Division of Ser. No. 286,625, Sept. 6,1972.

US. Cl 29/599, 174/126 CP, 174/128, l74/DIG. 6

Int. Cl HOlv 11/14 Field of Search..... 29/599; 174/114 S, 126 CP,174/128, 129 S, DIG. 6

References Cited UNITED STATES PATENTS l/l937 Gilbert 174/128 X 12/1970Albrecht 174/128 1l/l971 Morton et a1 29/599 4/1972 Woolcock et al174/126 CP 3,699,647 10/1972 Bidault et a1. 29/599 3,714,371 1/1973Nomura et a] 174/126 CP 3,760,093 9/1973 Pemberton 174/128 FOREIGNPATENTS OR APPLICATIONS 708,162 7/1931 France 174/114 S 977,584 5/1967Germany 174/129 5 Primary Examiner-Charles W. Lanham AssistantExaminerD. C. Reiley, Ill

Attorney, Agent, or F [rm-Larry R. Cassett and H. Hume Mathews [5 7ABSTRACT 7 Claims, 9 Drawing Figures PATENTEUnuszo 1974 FIG. 1

FIGA

FIG.3

FIG. 7

FIG. 6

FIG.9

MULTI-FILAMENT COMPOSITE SUPERCONDUCTOR WITH TRANSPOSITION OF FILAMENTSAND METHOD OF MAKING SAME This is a division of application Ser. No.286,625 filed Sept. 6, 1972.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates generally to superconductors and more specifically tomulti-filamentary superconductor composites that are intrinsically verystable.

2. Prior Art A significant factor in the development of superconductorsand superconducting devices is the attainment of adiabatic stability.This is essential in order to attain reliable performance insuperconducting devices. Excellent superconducting properties have beenattained in laboratory testing of short samples. However, whensuperconducting devices were fabricated from long superconductor wire,performance comparable to short sample testing was not attained.

Failure to obtain short sample performance in superconducting deviceshas been attributed, in part, to flux jumps. Flux jumps causedegradation in superconducting devices by creating sudden local releasesof energy resulting in a premature transition to the normal state andthereby preventing reliable attainment of high current densities.

It is theorized that flux jumps occur when an external field is appliedto a superconducting device inducing loops of current at the criticalcurrent density which are unstable and may suddenly decay rapidly with arelease of energy. This sudden release of energy causes a transition inthe superconducting device from the superconducting state to the normalor resistive state thereby causing degradation of the superconductor.This sudden reversal can result in serious physical damage to thesuperconducting device.

Various techniques have been developed to improve the stability ofsuperconductors by minimizing degradation caused by flux jumping. In thepaper, Multifilamentary Superconducting Composites, by P. R. Critchlow,E. Gregory and B. Zeitlin, published in Cryogenics, February 1971, pp.3-10, recent developments that have contributed to the production ofstable superconducting composites are discussed. One such development isthe production of superconductors from composites having 50-1 000superconducting filaments in a matrix of high purity copper. However,the apparent advantages offered by miltifilamentary composites were notfully realized because these wires behaved in some respects essentiallythe same as an equivalent solid single core conductor and the occurrenceof flux jumping was unchanged.

This paper points out a technique whereby flux jumps induced by anexternal field can be reduced and the full advantages ofmultifilamentary composites attained. If the composite is twisted, thewire is effectively cut into lengths equal to half the twist pitch. Thetwisted wire should, therefore, behave as a collection of isolatedfilaments and consequently be more stable than a comparable untwistedmultifilament composite.

Critchlow et al disclose that a twisted composite is still susceptibleto degradation and loss from self-field effects. The self-field of thewire is produced by the transport current flowing in it. This results inan unequal current distribution in which the outer filaments carry morecurrent than the inner ones. A flux jump may occur thereby producingmore uniform current distribution with a concomitant transition to thenormal state.

Twisting alone will not eliminate self-field effects because thefilaments never change their radial position. Transposition will arrangethe individual filaments so that they occupy successively every positionof the cross-section. The filaments in an individual wire cannot betransposed. However, by winding a number of these wires into a braid orcable, a transposed conductor can be constructed.

By forming a braid or cable insures that the inner wires get to theoutside of the conductor cross-section. These techniques, however, arenot entirely satisfactory because rigidity and high packing factorcannot be attained. Rigidity or wires in a superconducting device isnecessary to prevent objectionable wire motion. The shape of braided orcabled superconducting wire does not produce a high packing factor. Thisrestricts the amount of wire that can be contained in a magnet therebyaffecting the size of the magnet.

Accordingly, the present invention provides a novel superconductorcomposite comprised of a plurality of segments and method of making samethat utilizes superconducting filaments embedded in a normal matrix. Thesegments are twisted so as to obviate eddy current type losses resultingfrom external magnetic fields and the individual filaments are alsotransposed so that they occupy the same relative radial position withinthe superconductor cross-section, thereby also avoiding or eliminatingself-field losses.

SUMMARY OF THE INVENTION An object of this invention is to provide anintrinsically stable milti-filament superconductor.

A further object of this invention is to provide an intrinsically stablemilti-filament supercondcutor composed of a plurality of segmentedcomposites.

Still a further object of this invention is to provide a superconductorthat is free from degradation resulting from self-field losses.

Still a further object of this invention is to provide a method forproducing intrinsically stable multifilament superconductors.

A further object of this invention is to provide a method for producingintrinsically stable multifilament superconductors composed of aplurality of segmented composites.

These and other objects are obtained by assembling a first compositecomposed of a plurality of superconducting rods in a normal matrix. Thisfirst assembly is then mechanically worked until the superconductor rodsare reduced to a filament of a diameter approximating the desired finaldiameter. The mechanically reduced composite is then twisted andsegmented. As used herein, the term segmented means to altermechanically the cross-section of the superconductor composite fromround to triangular or rectangular by passing the composite through aforming device. A sec ond composite of rectangular or circularcross-section is made by assembling a plurality of the previously formedsegments. This second composite is then mechanically worked until thediameter of the filaments is reduced to approximately 0.3 to L4 mil. Themechanically worked composite is then twisted a second time.

The individual segments constitute composites composed of a suitablenumber of superconducting filaments in a matrix of normal materialhaving a twist of the necessary pitch. The segments are resistant toflux jumping induced by an external magnetic field and are therebystabilized against this form of degradation. However, the filamentarystrands within each individual segment will not vary in their radiallocation from the center of such segment and the segment will not avoidinstability resulting from generated self-fields. The individualsegments are combined to form a second composite. The second compositeis then mechanically worked and twisted to a predetermined pitch. Theconvoluted filaments contained within each segment of this secondcomposite assume a path of varying radial position along their lengthsfrom the center of the final conductor. Such varying distribution of theindividual filaments produces a more uniform sharing of each filament ofthe cross-section of the composite conductor so that each filamentshares substantially equally in the conducting current. Thus thetendency towards instability resulting from self-generated fields isavoided.

The advantages of the superconductor of this invention will be apparentfrom the following drawings and detailed descriptions in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS FIG. I is a diagrammatictransverse schematic of a first composite of this invention consistingof superconductor rods in a normal matrix.

FIG. 2 is a diagrammatic longitudinal schematic of a first composite ofthis invention showing the position of an outside filament aftertwisting.

FIG. 3 is a diagrammatic transverse schematic of a first composite aftersegmenting.

FIG. 4 is a diagrammatic transverse schematic of another embodiment of afirst composite after segmentmg.

FIG. 5 is a diagrammatic transverse schematic of a second compositeconsisting of a plurality of triangular segments. FIG. 6 is adiagrammatic longitudinal schematic of a second composite showing theposition of a filament within the composite.

FIG. 7 is a diagrammatic transverse schematic of another embodimentshowing a second composite consisting of a plurality of rectangularsegments.

FIG. 8 is a diagrammatic transverse schematic of a second compositeafter twisting.

FIG. 9 is a diagrammatic longitudinal schematic of a second compositeshowing the position of a transposed filament within a segment aftertwisting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I,there is shown a superconducting composite 10 consisting of a normalmatrix 12 such as OFHC copper and superconducting alloy rods 14distributed therein. The superconducting alloy can be, for example,niobium-titanium containing approximately 50 percent of each element.After assembly, the composite can be fabricated in a manner well knownin the art, for example, extrusion rolling, drawing, swaging and heattreatment are well known processing techniques utilized to obtain acomposite of a specified reduced cross-section and optimumsuperconducting properties. In this instance composite 10 is reduced toa diameter approximating the desired final diameter.

After fabrication, composite 10 is twisted. As shown in FIG. 2, aftertwisting representative filament 14a follows path 16. The pitch or rateof twist is dependent upon the rate at which the superconducting devicewill be charged. For most applications a pitch of A: to 5 twists perinch is acceptable. The intermediate diameter must be of such amagnitude so that reduction to the final diameter will not remove theoriginal twist by elongating the wire. Generally speaking, thisintermediate diameter must not be more than twice the final desireddiameter.

FIGS. 3 and 4 show two different embodiments, 10a and 10b, of segmentedcomposite 10. After composites 10a or 10b are mechanically worked to anintermediate diameter and twisted, they are then segmented by mechanicalshaping. Shaping to the desired cross-section may be performed by usingdies, grooved rolls or a Turks-head. A Turks-head consists of 4-hardenedsteel rolls set in planes at right angles to each other. The narrow faceof the rolls, as set in the framework, is adjustable on the same planeso that the assembly of the overlapping roll edges facing each otherwill project a contour of the opening so formed, into the desired shapeof the cross-section of the product to be made.

Preferably the angle 0 in FIG. 3 should be a subdivision of 360, Le, l0,l2, 15, 20, 24, 30, 45, 60, 72, 90, 120, etc., so that the triangular orrectangular segments 10a or 10b so produced by the Turks-head can beassembled to form a second composite 22 or 30 as shown in FIGS. 5 and 7,respectively. After mechanically forming segments 10a or 1012, thesegments can be further treated in order to facilitate assembly ofcomposites 22 or 30. The segments may be passed through a solder bath ofsilver-tin (3% Ag, 97% Sn). This coating of solder will hold thesegments together during further processing. The assembly can also beformed by ultrasonically welding the segments together.

FIG. 5 shows a second composite 22 composed of triangular segments 20a,20b, 20c, 20d, 20e, 20f, 20g and 20h. The angle 6formed during drawingfirst composite 10 through the forming dies is 45". As shown in FIG. 6,representative filament 24 in segment 20a is transposed along the path26.

FIG. 7 shows another embodiment of a second composite 30 composed ofrectangular segments 32a, 32b, 32c and 32d.

As shown in FIGS. 5 and 7, after assembly the mating segments have acommon junction point, 28 and 34, which is located at the center of thesecond composite.

This places some of the filaments which were formerly situated at theoutside of the original composite on the inside or near junction point28 or 34 of the second composite. These filaments were originallytwisted and they will retain their position in the interior of thesegment cross-section after assembly of the second composite.Furthermore, filaments originally located near the center of thesegments, such as filament 24, will now be located near the outside ofthe second composite. When the second composite is twisted afterassembly, the heretofore centrally located filaments will now betransposed and occupy varying positions within the superconductorcross-section.

FIG. 8 shows second composite 22 after mechanical reduction to finalsize. The multi-filaments contained within segments 20a through 20h arenot shown for reasons of clarity. However, the diameter of thesefilaments is approximately 0.3-0.4 mil in diameter. After finalreduction the composite may be heat treated in a manner well known inthe art in order to obtain a metallurgical micro-structure that willproduce the optimum superconducting properties.

FIG. 9 shows the segmented second composite after twisting. Thistwisting serves to decouple the filaments clectromagnetically ashereinbefore discussed. The rate of twist is represented by numerical 36in FIG. 8. The segmented composite is now stabilized against selfinducedflux jumps. Representative filament 24 in segment 20 a is transposedalong the path 27.

Superconducting wire produced by the novel method of this invention hasseveral distinct advantages over superconducting wire produced byconventional prior art methods. Some of the more apparent advantagesinclude: I

A solid superconducting wire of equivalent diameter and equalsuperconductor alloy to normal material ratio will exhibit more lossesthan a segmented superconductor composite produced by the inventiondisclosed herein. The windings of superconducting devices, such asmagnets, will be more rigid with less wire motion when wound with thisnovel segmented wire as compared to other intrinsically stablesuperconducting wires, such as braid or cable. Furthermore, the shape ofthis wire produces a higher packing factor than braid or cable,resulting in more wire per superconducting device. Still further, acomposite with a like number of small diameter filaments merely twistedand not transposed will not be stable against self-induced losses andthe advantages of small diameter filaments will not be fully realized.

SPECIFIC EXAMPLE First Composite A first composite was assembled byinserting 360 rods of a Nb-Ti alloy (Containing 55 percent by weight Nb)into an eight inch diameter copper billet. This composite was thenmechanically worked until the diameter of the composite wasapproximately 11 mil.

The mechanically worked composite was then twisted. The rate of twistwas from about one-half turns to five turns per linear inch. SegmentingFirst Composite The ll mil composite was then passed through aTurks-head. The rolls of the Turks-head were adjusted to produce asquare wire approximately mil square.

Second Composite The square segment was then passed through a bath ofsilver-tin solder (3% Ag, 97% Sn). A second composite was formed bypaying out four solder-coated segments from four bobbins. This composite30 is shown in FIG. 7. The payed out segments were passed through aseries of closely-spaced aligning dies. The properly aligned segmentsthen passed through a tapered die wherein a square composite was formed.At some convenient location heat was applied to the composite, meltingthe solder and bonding the segments together. The formed composite wastwisted at a rate from about one-half turns to about five turns perlinear inch.

The finished intrinsically stable composite contained four segments andwas approximately 20 mil square. The individual segments each contained360 filaments approximately 0.3-0.4 mil in diameter.

From the foregoing, it is apparent that by the present invention therehas been provided a particularly advantageous method for producing anovel intrinsically stable superconducting wire. The invention has beendescribed withreference to a presently preferred embodiment, however, itis intended to cover such modifications as fall within the spirit andthe scope of the invention as hereinafter claimed.

We claim:

1. A method for producing intrinsically stable superconducting wirecomprising the steps:

a. assembling a first composite by inserting rods of a superconductivealloy in a matrix of normal material;

b. working said composite so as to substantially reduce the diameter ofsaid composite and form filaments of said superconductive alloy rods;

c. twisting said composite at a rate from about onehalf turns to aboutfive turns per linear inch of said composite;

d. forming said composite into a segment by passing said compositethrough forming rolls;

e. assembling a second composite by arranging a plurality of saidsegments in a geometric configuration; and

f. twisting said second composite so as to transpose saidsuperconductive filaments so that the filaments substantially shareequally the cross-section of said segments.

2. A method as recited in claim 1 wherein said segments are bondedtogether before twisting said second segment.

3. A method as recited in claim 2 wherein said segments are passedthrough a molten bath of solder.

4. A method as recited in claim 2 wherein said segments areultrasonically welded together.

5. A method as recited in claim 1 wherein said segments are traingularin cross-section.

6. A method as recited in claim 1 wherein said segments are rectangularin cross-section.

7. A method as recited in claim 1 wherein said second composite istwisted at a rate from about one-half turn to about five turns perlinear inch.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. {8233061- Inventor(s) I It is certified 7 that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

I 1. Co1. 2, line 18, "or" should be --of-- 2. Col. '2,"11ne 67, "1.4"should be --o.

Signed and sealed this 21st day of January 1975.

(SEAL) Air-test:

' MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents USCOMM-DC 60376-P69 R U.S. GOVERNMENT PRINTING OFFICE [9'90-356-334 F ORM PO-105O (10-69) UNITED STATES PATENT OFFICE- CERTIFICATEOF CORRECTION Dated August Patent No 1 829 06,

Inventor(s) h I It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

*1. Col. 2 line 18, "or" should be --of-- 2. Col. 2,?1ine 67, "1.4"should be 0.4-!-

Signed andvsealed this 21st day of January 1975.

( SEAL Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner ofPatents USCOMM'DC 60375-P69 U.S. GOVERNMENT PRINT NG OFFICE 19690-366-334 F ORM PO-105O (10-69)

1. A method for producing intrinsically stable superconducting wirecomprising the steps: a. assembling a first composite by inserting rodsof a superconductive alloy in a matrix of normal material; b. workingsaid composite so as to substantially reduce the diameter of saidcomposite and form filaments of said superconductive alloy rods; c.twisting said composite at a rate from about one-half turns to aboutfive turns per linear inch of said composite; d. forming said compositeinto a segment by passing said composite through forming rolls; e.assembling a second composite by arranging a plurality of said segmentsin a geometric configuration; and f. twisting said second composite soas to transpose said superconductive filaments so that the filamentssubstantially share equally the cross-section of said segments.
 2. Amethod as recited in claim 1 wherein said segments are bonded togetherbefore twisting said second segment.
 3. A method as recited in claim 2wherein said segments are passed through a molten bath of solder.
 4. Amethod as recited in claim 2 wherein said segments are ultrasonicallywelded together.
 5. A method as recited in claim 1 wherein said segmentsare traingular in cross-section.
 6. A method as recited in claim 1wherein said segments are rectangular in cross-section.
 7. A method asrecited in cLaim 1 wherein said second composite is twisted at a ratefrom about one-half turn to about five turns per linear inch.